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       1                 :            : /*
       2                 :            :  * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
       3                 :            :  *
       4                 :            :  * Licensed under the Apache License, Version 2.0 (the "License").
       5                 :            :  * You may not use this file except in compliance with the License.
       6                 :            :  * A copy of the License is located at
       7                 :            :  *
       8                 :            :  *  http://aws.amazon.com/apache2.0
       9                 :            :  *
      10                 :            :  * or in the "license" file accompanying this file. This file is distributed
      11                 :            :  * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
      12                 :            :  * express or implied. See the License for the specific language governing
      13                 :            :  * permissions and limitations under the License.
      14                 :            :  */
      15                 :            : 
      16                 :            : #include <stdint.h>
      17                 :            : 
      18                 :            : #include "tls/extensions/s2n_ems.h"
      19                 :            : #include "tls/s2n_tls.h"
      20                 :            : #include "utils/s2n_safety.h"
      21                 :            : 
      22                 :            : static int s2n_client_ems_recv(struct s2n_connection *conn, struct s2n_stuffer *extension);
      23                 :            : static bool s2n_client_ems_should_send(struct s2n_connection *conn);
      24                 :            : 
      25                 :            : /**
      26                 :            :  *= https://www.rfc-editor.org/rfc/rfc7627#section-5.1
      27                 :            :  *#
      28                 :            :  *#   This document defines a new TLS extension, "extended_master_secret"
      29                 :            :  *#   (with extension type 0x0017), which is used to signal both client and
      30                 :            :  *#   server to use the extended master secret computation.  The
      31                 :            :  *#   "extension_data" field of this extension is empty.  Thus, the entire
      32                 :            :  *#   encoding of the extension is 00 17 00 00 (in hexadecimal.)
      33                 :            :  **/
      34                 :            : const s2n_extension_type s2n_client_ems_extension = {
      35                 :            :     .iana_value = TLS_EXTENSION_EMS,
      36                 :            :     .is_response = false,
      37                 :            :     .send = s2n_extension_send_noop,
      38                 :            :     .recv = s2n_client_ems_recv,
      39                 :            :     .should_send = s2n_client_ems_should_send,
      40                 :            :     .if_missing = s2n_extension_noop_if_missing,
      41                 :            : };
      42                 :            : 
      43                 :            : static int s2n_client_ems_recv(struct s2n_connection *conn, struct s2n_stuffer *extension)
      44                 :       7583 : {
      45 [ -  + ][ #  # ]:       7583 :     POSIX_ENSURE_REF(conn);
      46                 :            : 
      47                 :            :     /* Read nothing. The extension just needs to exist without data. */
      48 [ -  + ][ #  # ]:       7583 :     POSIX_ENSURE(s2n_stuffer_data_available(extension) == 0, S2N_ERR_UNSUPPORTED_EXTENSION);
      49                 :       7583 :     conn->ems_negotiated = true;
      50                 :            : 
      51                 :       7583 :     return S2N_SUCCESS;
      52                 :       7583 : }
      53                 :            : 
      54                 :            : /**
      55                 :            :  *= https://www.rfc-editor.org/rfc/rfc7627#section-5.3
      56                 :            :  *= type=exception
      57                 :            :  *# When offering an abbreviated handshake, the client MUST send the
      58                 :            :  *# "extended_master_secret" extension in its ClientHello.
      59                 :            :  *
      60                 :            :  *  We added an exception here in order to prevent a drop in 
      61                 :            :  *  session resumption rates during deployment. Eventually clients
      62                 :            :  *  will be forced to do a full handshake as sessions expire and pick up EMS at that point.
      63                 :            :  **/
      64                 :            : static bool s2n_client_ems_should_send(struct s2n_connection *conn)
      65                 :       7724 : {
      66                 :            :     /* Don't send this extension if the previous session did not negotiate EMS */
      67 [ +  - ][ +  + ]:       7724 :     if (conn && conn->set_session && !conn->ems_negotiated) {
                 [ +  + ]
      68                 :        129 :         return false;
      69                 :       7595 :     } else {
      70                 :       7595 :         return true;
      71                 :       7595 :     }
      72                 :       7724 : }